Nonsense. Choosing the correct database isn't "premature optimization", it's the bare minimum of being a competent programmer. Why do we never have time to do it right, but we always have time to do it over?
If you're talking about a startup, you might not even get to the let's do it over stage. You might have six months to just hack something out so you can raise more funding, and in that case Mongo is a great choice.
The needs of your business might also just evolve over time.
